Digital monitoring for length variation of optical fibers in femtosecond timing and synchronization system of XFEL

Digital monitoring for length variation of optical fibers in femtosecond timing and synchronization system of XFEL Abstract:The requirements for the femtosecond timing and synchronization (fs TS) system in an X-ray free electron laser (XFEL) facility are analyzed. The timing/phase distribution method using optical fiber is adopted. Since the optical length of the fiber changes as the temperature drifts, the influence of temperature drift on the length variation of the fiber is studied in comparative experiments. An FPGA-based digital phase/amplitude detector is designed to record the data of the length variation. For a 100-meter-long fiber, the timing drift caused by length variation is 6 ps (peak-peak value) over 24 hours. The result shows the digital phase/amplitude detector could be used for length variation monitoring and stabilization of the fs TS system.